We can clearly see this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can
be supported in any country has long ago arrived at its full average.
If its natural powers of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in its conditions)
only by its varying descendants seizing on places at present occupied
by other animals: some of them, for instance, being enabled to feed on
new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less
carnivorous. The more diversified in habits and structure the
descendants of our carnivorous animal became, the more places they
would be enabled to occupy. What applies to one animal will apply
throughout all time to all animals--that is, if they vary--for
otherwise natural selection can do nothing. So it will be with plants.
It has been experimentally proved, that if a plot of ground be sown
with one species of grass, and a similar plot be sown with several
distinct genera of grasses, a greater number of plants and a greater
weight of dry herbage can thus be raised. The same has been found to
hold good when first one variety and then several mixed varieties of
wheat have been sown on equal spaces of ground. Hence, if any one
species of grass were to go on varying, and those varieties were
continually selected which differed from each other in at all the same
manner as distinct species and genera of grasses differ from each
other, a greater number of individual plants of this species of grass,
including its modified descendants, would succeed in living on the
same piece of ground. And we well know that each species and each
variety of grass is annually sowing almost countless seeds; and thus,
as it may be said, is striving its utmost to increase its numbers.
Consequently, I cannot doubt that in the course of many thousands of
generations, the most distinct varieties of any one species of grass
would always have the best chance of succeeding and of increasing in
numbers, and thus of supplanting the less distinct varieties; and
varieties, when rendered very distinct from each other, take the rank
of species.

The truth of the principle, that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be severe, we always find great diversity in its
inhabitants. For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly the
same conditions, supported twenty species of plants, and these
belonged to eighteen genera and to eight orders, which shows how much
these plants differed from each other. So it is with the plants and
insects on small and uniform islets; and so in small ponds of fresh
water. Farmers find that they can raise most food by a rotation of
plants belonging to the most different orders: nature follows what may
be called a simultaneous rotation. Most of the animals and plants
which live close round any small piece of ground, could live on it
(supposing it not to be in any way peculiar in its nature), and may be
said to be striving to the utmost to live there; but, it is seen, that
where they come into the closest competition with each other, the
advantages of diversification of structure, with the accompanying
differences of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general rule,
belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which have succeeded in becoming naturalised in any land would
generally have been closely allied to the indigenes; for these are
commonly looked at as specially created and adapted for their own
country.